This invention relates to a signal detector which detects an input signal for measuring, for example, the frequency of the input signal, and especially to a signal detector which detects a frequency component having the largest amplitude among a plurality of frequency components contained in the input signal.
In measuring a frequency component of an input signal whose frequencies are in a microwave or higher band, a direct count by a frequency counter for instance is not possible, or, when it is possible, an extremely expensive counter is required. For avoiding this difficulty, generally, a counter which includes a frequency converter is employed and the input signal is mixed with a local signal so as to be converted to an intermediate frequency signal having a frequency which can be counted by a ordinary circuit arrangement.
The frequency converter in the counter usually includes a harmonic mixer for obtaining intermediate frequency signals not only of a sum and a difference of the fundamental frequency of the input signal and the local signal, but also sums and differences of the frequency of the input signal and the harmonic frequencies of the local signal.
One of the known frequency measuring apparatuses employing a frequency converter is illustrated in FIG. 1. A local signal from a local oscillator 33 and an input signal from a terminal 31 are fed into a frequency converter 32, whereby these are mixed with each other, and intermediate frequency signals are produced having frequencies equal to the sum or the difference between the frequencies of the input and local signals. The frequency of the local signal is linearly changed by a sweep signal from a sweep generator 44. The intermediate frequency signal is amplified by an amplifier 34 which has the characteristics of a relatively wide bandpass filter.
An amplitude detector 35 determines whether the amplitude of the intermediate frequency signal from the amplifier 34 is higher than a predetermined voltage level. If the intermediate frequency signal exceeds the predetermined voltage level, the amplitude detector 35 generates a detection signal to provide it to a control circuit 43. When the detection signal is generated, the sweep operation of the local oscillator 33 is temporarily stopped by a sweep stop command from the control circuit 43 and the frequencies of both the local signal and the intermediate frequency signal are measured. Namely, the local signal and the intermediate signal are gated by gate circuits 36 and 37 whose other input terminals are provided with a gate signal from a gate signal generator 38, and the number of pulses in the gated signals are counted by the counters 39 and 40.
After the measurement, the control circuit 43 provides a sweep restart command to the sweep generator 44, so that the frequency of the local oscillator 33 is again linearly changed a little. After a small change of the local frequency, the sweep operation is again stopped by the sweep stop command, and the frequencies of both the local and intermediate frequency signals are again counted by the counters 39 and 40. Namely, the frequency measurement occurs at least twice in a certain small time interval with slightly different local frequencies under the control of the control circuit 43. The counted values in the first and second measurement are stored in a calculator 41 so as to calculate the frequency of the input signal detected by the amplitude detector 35.
Let it be assumed that the input signal frequency is F.sub.x, the local signal frequencies counted in the first and second measurements are F.sub.L1 and F.sub.L2, respectively, the intermediate frequency signal frequencies counted in the first and second measurements are F.sub.i1 and F.sub.i2, and the harmonic number of the local signal is N. In the case where the input signal frequency F.sub.x is higher than the local signal frequency, the following relations are obtained. EQU F.sub.x =NF.sub.L1 +F.sub.i1 (1) EQU F.sub.x =NF.sub.L2 +F.sub.i2 (2) EQU N =-( F.sub.i1 -F.sub.i2)/( F.sub.L1 -F.sub.L2) (3)
Therefore, the harmonic number N of the local signal is obtained by equation (3) and the input signal frequency F.sub.x can be calculated by equation (1) or (2) by applying the harmonic number N. A calculator 41 calculates the input frequency F.sub.x according to equations (1) to (3) with use of the counted values obtained in the first and second measurements. The input signal frequency thus obtained can be displayed by a display 42.
In this frequency measuring apparatus, since the amplitude detector 35 generates a detection signal when an intermediate frequency signal exceeds the predetermined voltage level, no correct frequency measurement can be expected when a spurious signal which has an amplitude larger than the predetermined voltage level is contained in the input signal. Namely, when the voltage level of the spurious signal at the amplitude detector 35 is higher than the predetermined detection level, the detector 35 generates a detection signal in synchronism with the occurrence of the spurious signal. As a result, the frequency of the spurious signal, instead of the input signal, is measured.
Input signals usually contain harmonic and non-harmonic spurious signals. In other words, generally there is a plurality of frequency components in an input signal, but frequently the true input signal that is to be detected and measured has the largest amplitude among the components. In this case it is necessary to detect the frequency component which has the largest amplitude among the plurality of frequency components contained in the input signal, for correctly measuring the desired signal.